1. Field of the Invention
The present invention relates to a tire air-pressure abnormality alarming device and a method thereof. More particularly, it relates to a tire air-pressure abnormality alarming device and a method thereof capable of maintaining normal operations for detecting decompression of a tire.
2. Description of Related Art
It might happen that air-pressure of a tire decreases due to unexpected factor such as a blowout or damage of a valve. When a vehicle continues running with a tire of decreased air-pressure, the tire is increasingly deformed and generates heat in the worst case, the tire may burst. Therefore, numerous devices for alarming abnormalities in tire air-pressure have been proposed in the past.
For instance, a method is known for detecting a decrease in internal pressure based on relative differences in the number of revolutions of tires (see Japanese Unexamined Patent Publication No. 149119/1995).
However, since variations in the diameter of tires having the same size are larger than the rate of change in the number of revolutions of the tires due to decompression of approximately 30%, it cannot be determined whether the internal pressure is normal or abnormal when a tire has been exchanged. Therefore, it is required to preliminarily store a relative difference or rate between the number of revolutions of tires when the internal pressures are normal at an initial stage if a tire has been exchanged.
A method has also been proposed for detecting a decrease in air-pressure based on changes in frequency characteristics of tire rotation (see Japanese Unexamined Japanese Patent Publication No. 133831/1993). In this method, when a tire or wheel has been exchanged, the frequency characteristics of tire rotation changes accompanied by a change in unspring weight, that is, the total weight of the tires and wheels, so that it is required to store resonant frequencies of a set of these tires and wheels at a normal internal pressure.
Several methods are known for starting operations for calculating and storing variations in tire diameters or resonant frequencies of tire rotations when the tires are at a normal internal pressure. For example, pressing a press button switch for more than a specified time, short-circuiting a specified terminal to IG or GND (grounding), stepping on the brake for a prescribed number of times within a specified time or continuously stepping thereon for a specified time, or combining some of these methods.
However, maintaining normal operations for detecting decompression of a tire becomes difficult in the following cases.
1. When initialization is started with a tire being in a decompressed condition:
A driver can recognize an abnormality of a tire when air pressure of a tire has decreased, a tire air-pressure abnormality alarming function is actuated and an alarm lamp is turned on. However, since an external appearance of a tire of which air-pressure has decreased by 30% from a prescribed initial pressure is hardly distinguishable from that of a tire of normal air-pressure since a limited number of vehicles are always equipped with air-pressure meters, the driver might erroneously consider that the internal pressure is correct and consider instead that the alarm lamp has turned on due to a failure of the alarming device.
In this case, an initializing switch might be pressed with the aim of turning the alarm lamp off in a hurry when a design in which initialization is started by pressing a press button switch is used. Consequently, initialization is started regardless of the fact that the air-pressure of a tire has decreased, and the alarm lamp is turned off.
Further, erroneous initialization is also started when the driver considers the air-pressure of a tire to be normal and short-circuits a specified terminal to a 12V line such as IG and BATT, or GND, or operates the brake in accordance with a special series of steps.
2. When initialization is to be started while the vehicle is running:
If initialization is started by pressing an initializing switch, pressing the initializing switch after start of driving means that the driver needs to perform an action that the driver usually does not take during driving. Therefore, displaying completion of acceptance such that the driver might recognize it when the driver has pressed the initializing switch during driving of the vehicle (e.g. blinking the lamp three times) will thus induce dangerous operations.
3. When initialization is to be started in an inspection mode:
At the time of inspection before shipping vehicles from factories, operation of the hardware such as an alarm lamp or switches are confirmed in an inspection mode which is different from normal operation.
However, when the essential initialization is started at the time of inspecting switches, it might happen that the set up of air-pressure is inappropriate or that an inappropriate coefficient is stored by rotating four wheels on a chassis dynamo.
4. When initialization is to be started when the device is broken or in a terminated condition:
A device which detects an abnormality in air-pressure of a tire based on the number of revolutions of tires of four wheels cannot correctly function when, for example, wheel speed pulses are not correctly input to the device. It is thus required to periodically check (e.g. at periods of 40 msec) the wheel speed input. There are known methods therefor such as observing input voltages through hardware or detecting broken lines of the wheel speed pulse input lines by comparing the number of revolutions of the tires of the four wheels through software.
If the driver has pressed the initializing switch for starting initialization just after breakdown of a wheel speed pulse has been detected through this self-diagnosis, the driver will erroneously consider that the device is functioning in a normal condition although it is not the case when the device displays completion of acceptance recognizable to the driver (e.g. the lamp blinking three times). Similarly, when a function of the device has been terminated due to malfunctions which frequently occur in the software or the like, displaying completion of acceptance upon pressing the initializing switch will make the driver erroneously consider that the device is functioning correctly though functions are in a terminated condition.
The present invention has been made in view of these facts, and it is an object thereof to provide a tire air-pressure abnormality alarming device and a method thereof which is capable of maintaining a correct function of detecting decompression of a tire by setting starting conditions for initializing operations for updating stored values for physical amounts related to tires or wheels.